A protective helmet of the above mentioned kind is previously known from the patent document EP-A1-1246548. This protective helmet is marketed under the name MIPS™. The MIPS™ protective helmet aim at reducing the energy that is transferred to the brain during oblique impacts. The protective helmet comprises an outer layer and an inner layer, also called “liner”. Between the liner and the outer layer there is one or more sliding layers, that allows the outer layer and the liner to be displaced with low friction relatively each other. For example, the sliding layer may compose of oil, air, TEFLON or the like. Additionally, the protective helmet comprises an energy-absorbing elastic connecting member that, during displacement of the outer layer relatively the “liner” during an oblique impact towards the helmet, is deformed and absorbs impact energy.
A protective helmet of the above-mentioned kind may for instance be designed such that the outer and inner layers and the sliding layer during a displacement are deformed, and thus such that the protecting ability of the helmet is consumed. With respect to the protective helmet according to the above-mentioned patent document EP-A1-1246548, the energy-absorbing elastic connecting members can be designed such that they are deformed and thus are consumed at a displacement of the layers. Under certain circumstances there is a need for a protective helmet which to some extent delimit the displacement of the layers relatively each other. Such a circumstance is for example that a relative displacement of the layers are not desired when the protective helmet is exposed for insignificant or low loads that do not imply any risk for brain damages of the helmet wearer. One example is some degree of careless handling e.g. during gentle impacts when the wearer has taken off the helmet and happens to drop it, from a height, in the floor. Another example is careless conveyance handling at delivery of newly-produced helmets.
If the helmet has been influenced such that an energy-absorbing part, alternatively an elastic connecting member, has been activated/deformed and thus has been consumed at a displacement of the layers, it is necessary to be able to fast and unambiguously establish, without any complicated inspection, that the helmet is consumed, or alternatively that the helmet have to be repaired or taken care of for resetting to its original unaffected condition.